1. Field of the Invention
This invention relates to a method of producing a master information carrier and a master information carrier for magnetic transfer carrying thereon information to be transferred to a slave medium.
2. Description of the Related Art
As a method of recording information on a magnetic recording medium, magnetic transfer has been used. In the magnetic transfer, the surface of a master information carrier having thereon an irregularity pattern (a pattern of protruding portions and recessed portions) of magnetic material representing information to be transferred is brought into close contact with a surface of a slave medium (a magnetic recording medium) having a magnetic layer and a transfer magnetic field is applied to the slave medium and the master information carrier in this state, thereby recording on the slave medium a magnetization pattern representing the information (e.g., a servo signal) on the master information carrier. See, for instance, Japanese Unexamined Patent Publication No. 63(1988)-183623 and U.S. Pat. Nos. 6,347,016 and 6,567,227.
The master information carrier generally comprises a substrate of, for instance, silicon or glass and an irregularity pattern of magnetic material formed on the substrate by photolithography, sputtering, etching or the like.
Further, the master information carrier may be produced by the use of lithography technology which has been used in producing semiconductors, stamper producing technology which has been used in producing optical disc stampers, or the like.
In order to improve the quality of the signal transferred by the magnetic transfer, it is necessary to accurately form a pattern of a magnetic layer on the master information carrier. It has been found that the shape of the protruding portions of the pattern varies according to the system for drawing the pattern and the magnetic transfer characteristics of the master information carrier are affected by the shape of the protruding portions of the pattern.
For example, in the case where the slave medium is in the form of a rotary magnetic recording disc, an irregularity pattern representing a servo signal comprises protruding portions which are of a square or a rectangle longer in the direction of width of the recording tracks (in a radial direction of the recording disc). The pattern is generally drawn by projecting a laser beam modulated according to the information to be transferred onto a disc-like substrate coated with photoresist while rotating the substrate.
However, as the track width is narrowed to not larger than 0.3 μm, for instance, to meet a demand for a higher recording density, the drawing diameter of a laser beam (the diameter at which a laser beam draws an image on the substrate) comes not to be able to be thinned to draw a pattern of protruding portions in such a narrow tracks. As a result, the protruding portions come to have arcuate end portions and cannot be rectangular in shape. Arcuate end portions of the protruding portions cause a recording loss (azimuth loss) in the transfer magnetic field applied to the master information carrier and the slave medium which are held in close contact with each other, which results in imperfect formation of a magnetization pattern on the slave medium, whereby the signal transferred to the slave medium becomes unsharp.